1. Field of the Invention
The present invention relates to an imaging apparatus installed, for example, on a vehicle, and more particularly, the invention relates to an imaging apparatus having a wide dynamic range that is capable of sensing images without video signals being saturated or painted out to a light side or a dark side even with respect to objects having a light and dark difference with a large dynamic range.
2. Description of the Related Art
In general, in imaging apparatuses, when a light and dark difference exceeds the dynamic range of an image sensor under the condition of an extremely large difference between a high brightness level and a low brightness level of an object, there arises a problem of the white paint-out of a high brightness portion or the black paint-out of a low brightness portion.
With the advent of an imaging apparatus capable of taking picture of an object in the external world with a wide dynamic range, however, it is becoming possible to acquire a video signal that holds intensity or brightness change information without generating either a white paint-out or a black paint-out even under the above situation.
But, when an external world of a wide dynamic range is sensed to provide a video signal by the use of the above-mentioned imaging apparatus, the dynamic range of the video signal itself is limited, so the video image thus obtained can not help but become relatively low in contrast. Particularly, in cases where video signals are used for image processing, there arises another problem that image processing becomes unstable resulting from that an amount of intensity or brightness change or an edge component becomes less prone to be obtained.
In addition, when a wide dynamic range is reproduced by digital values, it is necessary to raise the resolution (the number of bits) of an A/D converter in accordance with the width of the dynamic range, thus giving rise to a further problem that the system becomes large in scale and high in cost.
For instance, assuming that an inexpensive general A/D converter has a resolution of 8 bits or less, an expensive A/D converter has a resolution of 9 bits or more.
Further, when considering that a dynamic range equivalent to the external world is to be reproduced, a similar value is also requested with respect to the S/N ratio of a video signal. Therefore, a very severe noise reduction is required, so there is also a problem that it is necessary to achieve an unrealistic S/N ratio from a design point of view.
Accordingly, an imaging apparatus has been proposed in which by arranging, as a countermeasure for the above-mentioned problems, a gradation correction device that performs gradation correction for signals with small amounts of exposure before synthesizing a plurality of signals with different amounts of exposure output from an image sensor, a satisfactory gradation characteristic can be obtained in high brightness portions of an image after the synthesis thereof by an image synthesis device (see, for instance, a first patent document: Japanese patent application laid-open No. 10-233966 (paragraph [0014])).
Also, another image processing apparatus has been proposed that is provided with an image data conversion device for emphasizing a change in brightness in a plurality of luminance or brightness ranges by using an image data translation table (see, for example, a second patent document: Japanese patent application laid-open No. 2002-312781 (paragraph [0007])).
In the above-mentioned conventional imaging apparatuses, for example in the case of the one described in the first patent document, a high-speed calculation part for performing arithmetic calculations for synthesis in real time is required for each of the gradation correction device and the image synthesis device, and hence there is a problem that the circuit size of the imaging apparatus becomes large.
In addition, in the case of the imaging apparatus described in the second patent document, there are the following problems. That is, a memory is required for holding the output translation table, thus resulting in a large circuit size. Besides, the output translation table has a unique characteristic, so desired video images or contrast can not be obtained with respect to sensed or picked-up scenes that are unable to suit the table characteristic.